[AArch64] Default to zero-cycle-zeroing FP registers

It is generally beneficial to prefer "movi d0, #0" over "fmov s0, wzr" as this
is most efficient across all cores; it is recognised as a zeroing idiom. For
newer cores, fmov instructions can also be eliminated early and there is no
difference with movi, but some implementations lack this so is not true for
other/older cores. Thus this standardises on using movi as this should always
gives the same or better performance than the fmov with wzr.

Differential Revision: https://reviews.llvm.org/D99586